An active-shield assembly is one in which an inner coil system and an outer coil system are connected in series to create magnetic fields in opposing directions so that a strong uniform magnetic field is created at the centre of the coil systems but the stray field prevailing outside the coil systems is very small.
Such an assembly is described in U.S. Pat. No. 4,587,504 issued on U.S. Ser. No. 669,311 filed Nov. 7, 1984, the contents of which are incorporated herein by reference. In the assembly described in U.S. Pat. No. 4,587,504, the coil systems creating the opposing magnetic fields are wound from superconducting wire. Superconducting coil systems are used in order to achieve strength and high precision of the magnetic field created in the centre of the coil systems. A prime application of these magnet assemblies is in NMR (nuclear magnetic resonance) imaging where these qualities are of vital importance.
In order to obtain an image of a subject using NMR techniques a steady background magnetic field is required which is uniform over the region to be imaged (the working volume).
Onto this uniform field are superimposed time-varying gradient magnetic fields which are used to encode position information into the image signal. A superconducting coil system is of particular advantage when the steady magnetic field strength is high, for example more than 0.3T, and the working volume is large, for example a sphere of 0.5 m diameter.
The superconducting condition of the coils is achieved by cooling the coils to very low temperatures, of the order of 4.2 K, using cryogenic techniques. In the superconducting condition, the wires can carry a very high current with negligible energy losses in the wire.
A conventional superconducting coil magnet (that is, a single coil system, without a balancing coil system as in an active shield assembly) has a further quality in that the magnetic disturbances to the magnetic field in the working volume due to external sources of magnetic field, such as other remote coil systems or magnetized objects, are automatically compensated to some degree. This arises from a quality of superconducting coils in that the current in a closed superconducting loop automatically changes in a sense to resist changes in the total magnetic flux threading the loop caused by external disturbing influences. The extent to which the magnetic disturbances are automatically compensated depends on, among other things, the geometry of the magnet and particularly its aspect ratio (the ratio of .sup.d /1 where d is the internal diameter of the magnet and 1 is its length). For a magnet having an aspect ratio of 2 or 3 such as might be used in an imaging application, the effect of an external disturbance is reduced by a factor of about 8 when compared with the field change that would occur if the magnet were absent. The theory underlying this self shielding effect is explained in more detail in U.S. Pat. No. 4,974,113 issued on Ser. No. 168,920 assigned to President and Fellows of Harvard College. This patent also describes arrangements of superconducting screening coils which are intended to reduce even further the effect of external disturbances.
However, a direct consequence of adding the outer coil system to form an active-shield assembly is that this automatic compensation is no longer effective in the working volume. The result is an increase in susceptibility to interference from external disturbing influences in an active-shield assembly as compared with a conventional single superconducting coil magnet. This interference is manifested by a distortion of the images which are produced by the interaction of the background and gradient fields in the working volume.
It is desirable to reduce the effect of disturbing influences on the magnetic field in the working volume of an active-shield assembly. In this regard it is important to consider not only the steady state condition of the assembly in which the current in the superconducting coils is substantially constant but also transient conditions in which the current is varying.
The most relevant of these transient conditions is a "quench" which is the term given for conversion of a coil from a superconducting state to a normal conducting state. A quench may occur unintentionally, due to local disturbances or structural deficiencies or it can be induced intentionally (for example, by means of local electrical heaters) as a way of rapidly reducing the magnetic field. This might be needed for example in a case where it is necessary to give urgent treatment to a patient undergoing an NMR procedure in the magnet assembly. When a quench occurs, there is a rapid increase in resistance in the quenched part of the coil which causes the energy stored in the coil system to be converted into heat. The heat is conducted to adjacent parts of the coil, causing these parts to quench. As an increasingly large part of the coil is quenched, the temperature associated with the heat energy increases, causing the resistance of the coil to rise further and accelerating the quench process until all the stored magnetic energy is converted into heat. This can occur extremely quickly, typically in about 10 to 20 seconds.
As described in U.S. Pat. No. 4,587,504 the coils of an active-shield magnet assembly are designed not only to produce a uniform magnetic field at the centre of the assembly but also to produce an external magnetic field which is as low as possible as close as possible to the assembly. The external magnetic field is termed herein the "stray field" and is commonly specified in terms of an ellipsoidal or cylindrical volume outside which the magnetic field due to the magnet assembly nowhere exceeds a specified level. This provides a way of denoting a safety zone a certain distance from the magnet. Any measures taken to reduce the effect of disturbing influences on the magnetic field in the working volume should preferably not be such as to cause the stray field to exceed its nominal limits, even during a quench.
U.S. Pat. No. 4,974,113 referred to above does not address specific problems associated with active-shield magnets or the difficulties arising during transient conditions.
An attempt to address these problems is proposed in EP-A-0299325 (Siemens) which describes an active-shield magnet in which the inner and outer magnet systems are decoupled by a superconducting current limiter. The current limiter permits slight current differences to exist between the inner and outer coil systems. As a result, magnetic field disturbances within each coil system can be compensated by a current modification. While the theory is sound, there are considerable difficulties in achieving a reliable practical implementation based on this theory. In particular, for magnets of a size and geometry suitable for use in NMR applications, the current limiter must have a limiting current of less than 1 amp yet be able to withstand several hundred volts applied across its end if the stray field is to be maintained within bounds, even during quench conditions. A typical critical current density in a superconductor is about 4000 A per mm.sup.2, so to achieve less than 1 amp it is necessary to use a very fine filament, of the order of 10 .mu.m. Such a filament is both difficult to manufacture and to handle. Moreover, as the ends of the filament are adjacent in the coil as wound, very high insulation is necessary between the ends because of the high voltages which are developed. Thus the problem outlined above remains unsolved at a practical level.